After holding another full day of labor negotiations, the NFL and its players will continue productive talks, but philosophical differences about some core issues remain, numerous sources with knowledge of the situation said Thursday.

For instance, significant work on how rookies will be compensated still must be done, according to multiple sources, with both parties at odds over how much negotiation will remain in the signing process and how much will be straight slotting, in which players will receive a preordained raise over the person drafted in their spot the previous year. The parties have worked on the issue for weeks but aren't seen as being near a consensus.

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"That one is still a real live wire," said one source involved in the process.

"Still a lot of work to be done there," another source said.

The parties haven't really started discussing how they will settle the pending litigation involved with the Brady et al v. National Football League et al antitrust case, which also is a potentially big hurdle, nor have they covered whether the NFL Players Association will reconstitute itself as a union and if federal judicial oversight will be involved in the future collective bargaining agreement.

There are issues involving veteran free agency, too. The owners have requested the right of first refusal to retain their existing free agents coming out of the lockout. But it's a divisive issue, termed by several sources as "a deal-breaker" from the players' perspective.

"That issue could break the whole thing up," one source said. "Players will not budge on it."

Ultimately, several sources opined that the owners would relent on that as the parties come closer to a deal, but for now, it remains a contentious issue that will continue to be addressed.

The owners are requesting that three offensive line designations (tackle, center, guard) be added to the franchise tag, whereas now there is only one offensive line designation across the board. Several sources believe that won't become a major stumbling block that would hold up a deal.

Although major gains have been made in creating a revenue model, there still are differences on whether certain forms of sales tax and charges would be included. This, too, is a relatively minor issue.

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The NFLPA conducted a conference call with the Brady lawsuit plaintiffs Thursday night, and it highlighted some of the significant differences that remain between the parties as negotiations continue into Friday.

"It was pretty much what you would expect," said one source with knowledge of the call. "It was an update on where things stand, that we're close on a lot of things, but close only counts in horseshoes and hand grenades."

U.S. Magistrate Judge Arthur Boylan, who's mediating these talks, leaves for vacation Saturday. Negotiations could be extended into the weekend, depending on how things progress Friday in New York, or tabled until next week.

"That's still fluid," one source said. "It might make sense to take 48 hours to regroup and then get together again next week. That's a fluid situation."